Artificial disc implant

ABSTRACT

An artificial disc implant includes an upper shell, a lower shell, and a spacer therebetween. The spacer preferably has properties similar to that of a natural spinal disc, while the upper and lower shells form a rigid interface between the implant and the adjacent vertebral bodies. The upper and lower shells can be configured to prevent expulsion of the spacer from the disc space. The implant upper and lower shells may further be configured into partially cylindrical shapes for ease of insertion through an insertion tube as presently known for interbody fusion devices. The devices may further be configured for insertion through a double-barreled insertion tube. Methods and instruments for inserting an artificial disc implant are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing date ofProvisional Application Serial No. 60/137,586 filed Jun. 4, 1999,entitled ARTIFICIAL DISC REPLACEMENT. The referenced application isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to artificial disc replacement devices.Previous attempts at artificial disc replacement have not received widespread acceptance because of a number of problems. In some attempts atdisc replacement, a flexible artificial disc is placed within theintervertebral disc space without any anchoring system, with theexpectation that the artificial disc will remain in place in the discspace based on contact with the ligaments of the disc annulus and/or thevertebral bodies. With this approach there remains an unacceptable rateof protrusion of the artificial disc from the disc space. Further, overtime the artificial disc may wear against the adjacent vertebralendplates, generating wear particles in the disc space and creating therisk of failure of the artificial disc.

Alternative designs have provided a rigid interface between thevertebral end plates and a shock absorbing compound disposed in the discspace between the rigid interfaces. The drawbacks of many of these priordevices are that they require extensive disc space preparation prior toplacement. Other attempts at disc replacement provide a device havingmultiple components that must be positioned in the disc space.

The present invention is directed to providing improved artificial discreplacement implants directed to solving a number of the problems anddisadvantages of the previous attempts at disc replacement.

SUMMARY OF THE INVENTION

The present invention provides an improved artificial disc implant forreplacing the spinal disc between two vertebrae of the spine. Theimplant comprises an upper shell, a lower shell and a spacer or inserttherebetween. Preferably the implant is insertable as a single unit intothe disc apace between two adjacent vertebral bodies. The shells may bemade from any suitable bio-compatible material.

According to one aspect of the invention, the upper and lower shellseach include a pair of interconnected cylindrical lobes. In one form,the upper and lower shells are partially cylindrical.

Further, the present invention contemplates insertion into the discspace via tubular instruments presently used for interbody fusionprocedures. Thus, the instrumentation utilized to perform currentinterbody fusion techniques may also serve a dual function for discreplacement procedures.

In another aspect of the present invention, there is provided an upperand lower shell for engagement with the vertebral bone of the adjacentvertebral bodies. The upper and lower shells each have anchoring meansto prevent movement in at least one direction. In one form, theanchoring means are ribs that prevent rotation of the shell in the discspace. In another form, each shell includes a flange extendingtherefrom. Each flange has an aperture extending therethrough receivinga bone screw to engage the shell to the adjacent vertebra.

Another aspect of the present invention, there is provided matingsurfaces between the upper and lower shells to restrict the transmissionof shear forces through the spacer disposed between the upper and lowershells. In one form, the mating surfaces are provided by multipleprojections and that are positionable in corresponding recesses torestrict movement in multiple directions while permitting compression ofthe spacer disposed between the upper and lower shells.

In still a further aspect of the present invention, shells forcontacting the upper and lower vertebral end plates are provided toanchor the device, and multiple spacer shapes are provided between theshells to permit insertion from a variety of approaches to the discspace, including anterior, posterior, lateral, anterior-lateral, andposterior-lateral approaches. The multiple spacer shapes are configuredto address a variety of angulations between the adjacent vertebrae.Various instruments and methods for insertion one or more artificialdisc implants to the disc space from a variety approaches are alsoprovided.

In yet another aspect of the present invention, the spacer includes twointerconnected cylindrical portions and the upper and lower shells havecavities shaped to securely retain the spacer therebetween.

In a further aspect of the invention, the spacer is inserted between theupper and lower shells. The sidewalls of the spacer are truncatedadjacent the gap between the upper and lower shells to limit potentialimpingement of the material between the upper and lower shells as thespacer is compressed.

In yet another aspect of the preferred invention, each of the upper andlower shells includes a substantially cylindrical bone engagementsurface for contact with a substantially cylindrical bone opening in thevertebral end plates. Preferably, the shells include structure to engagethe bone beyond the opening to limit movement of the shells in at leastone direction.

According to another aspect of the present invention, the implantincludes a spacer formed from a hydrogel substance. In one method ofinserting the implant according to the present invention, the hydrogelis at least slightly dehydrated thereby reducing the height of theimplant for insertion. Once inserted, the hydrogel can be hydrated toincrease the overall height of the implant to the desired workingheight.

These and other aspects, features, embodiments, forms, and advantages ofthe invention will become apparent from the following description of theillustrated embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an artificial disc implant according to thepresent invention.

FIG. 2 is a side view of the artificial disc implant of FIG. 1 rotated90 degrees about its axis.

FIG. 3 is an end view of the implant of FIG. 1.

FIG. 4 is a partial cross-sectional view of the implant of FIG. 2 takenalong line 4—4 with spacer removed.

FIG. 5 is a side view of a spacer for the implant of FIG. 1.

FIG. 6 is an end view of the spacer of FIG. 5.

FIG. 7 is a perspective view of another embodiment of an artificial discimplant according to the present invention.

FIG. 8 is an end view of the implant of FIG. 7.

FIG. 9 is a side view of the implant of FIG. 7.

FIG. 10 is an end view of a shell comprising a portion of the implant ofFIG. 7.

FIG. 11 is a top view of the shell of FIG. 10.

FIGS. 12 and 12a are each cross-sectional views of the shell taken alongline 12—12 of FIG. 11.

FIG. 13 is a cross-sectional view of the shell taken along line 13—13 ofFIG. 11.

FIG. 14 is a side view of the shell of FIG. 10.

FIG. 15 is a perspective view of still another embodiment of anartificial disc implant according to the present invention.

FIG. 16 is an end view of the implant of FIG. 15.

FIG. 17 is a side view of the implant of FIG. 15.

FIG. 18 is an end view of a shell comprising a portion of the implant ofFIG. 16.

FIG. 19 is a top view of the shell of FIG. 18.

FIG. 20 is a cross-sectional view of the shell taken along line 20—20 ofFIG. 19.

FIG. 21 is a cross-sectional view taken of the shell along line 21—21 ofFIG. 19.

FIG. 22 is a side view of the shell of FIG. 18.

FIG. 23 is a perspective view of a further embodiment of an artificialdisc implant according to the present invention.

FIG. 24 is a perspective view of a spacer utilized in the implant ofFIG. 23.

FIG. 25 is an end view of an alternate form of the implant of FIG. 23.

FIG. 26 is a top view of the implant of FIG. 23.

FIG. 27 is a cross-sectional view of the implant taken along line 27—27of FIG. 25.

FIG. 28 is a perspective view of another embodiment of an artificialdisc implant according to the present invention.

FIG. 29 is a side view of the implant of FIG. 28 with the externalthread pattern not shown.

FIG. 30 is an end view of the implant of FIG. 29.

FIG. 31 is a cross-sectional view taken along line 31—31 of FIG. 30.

FIG. 32 is a cross-sectional view taken along line 32—32 of FIG. 29.

FIG. 33 is a perspective view of a further embodiment artificial discimplant according to the present invention.

FIG. 34 is a perspective view of yet a further embodiment of anartificial disc implant having a central extension to limit movement inat least one direction.

FIG. 35 is a perspective view of still a further embodiment artificialdisc implant according to the present invention having a configurationto limit shear forces in the spacer.

FIG. 36(a) is a cross-sectional view similar to FIG. 32 showing anartificial disc implant having a spacer with truncated side wallsadjacent the separation between the upper and lower shells.

FIG. 36(b) is a perspective view of the spacer shown in cross-section inFIG. 36(a).

FIG. 37 shows the implant of FIG. 36(a) in a compressed state with thetruncated side walls compressed to make contact between the upper andlower shells.

FIG. 38 is another embodiment of an artificial disc implant according tothe present invention having a rectangular or square upper and lowershells.

FIG. 39 is the implant of FIG. 38 having upper and lower shellsconfigured to limit shear forces in the spacer.

FIG. 40 is another embodiment of an artificial disc implant according tothe present invention having substantially circular upper and lowershells.

FIG. 41 is the implant of FIG. 40 having upper and lower shellsconfigured to limit shear forces in the spacer.

FIG. 42 is a cross-sectional view of the relaxed state of asubstantially cylindrical artificial disc implant according to thepresent invention having a spacer disposed between upper and lowerpartial cylindrical shells.

FIG. 43 shows the implant of FIG. 42 in a compressed state with theupper and lower shells positioned closer to each other.

FIG. 44 is a cross-sectional view of a relaxed state of an artificialdisc replacement implant according to the present invention having upperand lower shells with substantially planar bone contacting surfaces anda spacer disposed therebetween.

FIG. 45 shows the implant of FIG. 44 in a compressed state.

FIG. 46 is an end view showing two of the implants of the presentinvention inserted into the disc space between two adjacent vertebralbodies.

FIG. 47 is a side view of the implants and disc space of FIG. 46.

FIG. 48 shows a partial perspective view of a disc space with an implantaccording to the present invention and being inserted by a substantiallylateral approach to a disc space.

FIGS. 49(a) and 49(b) illustrate an implant having spacers shapedaccording to the present invention for insertion into the disc spacefrom an anterior approach.

FIGS. 50(a) and 50(b) illustrate an implant having spacers shapedaccording to the present invention for insertion into the disc spacefrom an antero-lateral approach.

FIGS. 51(a) and 51(b) illustrate an implant having spacers shapedaccording to the present invention for insertion into the disc spacefrom a substantially lateral approach.

FIG. 52 is an end view of another embodiment artificial disc implantaccording to the present invention.

FIG. 53 is a top view of the artificial disc implant of FIG. 52.

FIGS. 54a-54 e illustrate various steps of a surgical techniqueaccording to the present invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any such alterations and furthermodifications in the illustrated devices, and any such furtherapplications of the principles of the invention as illustrated thereinare contemplated as would normally occur to one skilled in the art towhich the invention relates.

The present invention is directed to improved artificial disc implantsused to replace the spinal disc in an animal subject. In one embodiment,the invention contemplates an insert or spacer made from elastomer orhydrogel having properties of elasticity similar to or equivalent to anatural spinal disc. The spacer or insert is disposed between, butpreferably not interconnected with, an upper shell and a lower shell,each of which contact and/or engage an adjacent vertebral body.

Referring now to FIGS. 1-6, there is shown a first preferred embodimentof an artificial disc implant according to one aspect of the invention.More specifically, implant 10 includes an upper shell 12, a lower shell14, and an intermediate insert or spacer 16. Spacer 16 preferably haselastic properties substantially equivalent to the natural elasticproperties of the human body's intervertebral disc. In the illustratedembodiment, upper shell 12 and lower shell 14 are substantiallyidentical; however, it is contemplated that there could be differencesbetween the upper and lower shells without deviating from the spirit andscope of the present invention. The shells may be formed of any suitablebio-compatible material. For example, but without limitation, the shellsmay be composed of stainless steel, titanium, polymers, carbon fiber,shape memory alloys, or porous material. In the description thatfollows, the description of upper shell 12 applies with like effect tolower shell 14.

In the illustrated embodiment, upper shell 12 is partially cylindricaland includes a bone contacting surface 36 and lower shell 14 ispartially cylindrical and includes a bone contacting surface 38, each ofwhich is substantially arcuate and extends about longitudinal axis 11 toform a substantially cylindrical surface. Bone contacting surface 36 isinterrupted by a number of ribs 18 a, 18 b and 18 c, collectivelyreferred to as ribs 18, and bone contacting surface 38 is interrupted bya number of ribs 19 a, 19 b and 19 c, collectively referred to as ribs19. In the illustrated embodiment, three such retention ribs 18 and 19are provided on each shell; however, a fewer number or greater number ofribs are also contemplated. While a straight uninterrupted rib 18, 19 isshown in the preferred embodiment, it is contemplated that otherretention mechanisms such as barbs, interruptions, scales, etc., may beutilized to assist in retention and engagement of the upper and lowershells with its adjacent vertebral bodies. Ribs 18 and 19 further resistrotation of implant 10 about its longitudinal axis 11 in the disc space.

As shown in FIG. 5 spacer 16 has a length L1 along longitudinal axis 11,and as shown in FIG. 6 has a cylindrical shape extending alonglongitudinal axis 11 with a diameter D1. Referring now to FIG. 2 uppershell 12 lower shell 14 each have a length L2 that is greater thanspacer 16 length L1. Upper shell 12 and lower shell 14 thus extendsubstantially beyond the spacer 16 length to form overhanging endportions 28 and 30 and overhanging end portions 35 and 37, respectively,as shown in FIGS. 1 and 4. Overhanging end portion 30 of upper shell 12includes an end wall 32, and a similar end wall 31 is included withoverhanging end portion 28. Overhanging end portion 37 of lower shell 14includes an end wall 34, and a similar end wall 33 is included withoverhanging portion 35. Upper shell 12 includes a tapered portion 26extending from end wall 32 and through ribs 18 and a correspondingtapered portion 24 on the opposite end. Lower shell 14 includes atapered portion 27 extending from end wall 34 and through ribs 19 and acorresponding tapered portion 25 on the opposite end. The taperedportions may assist in easing insertion of the device through aninsertion apparatus, such as a sleeve, and into the disc space.

Referring to FIG. 4, upper shell 12 includes the combination ofpartially cylindrical interior bearing wall 48 and end walls 31 and 32to create an interior cavity or chamber 44 adapted to receive at least aportion of spacer 16 therein. Lower shell 14 similarly includes thecombination of preferably cylindrical interior bearing wall 49 and endwalls 33 and 34 to create an interior cavity or chamber 46 adapted toreceive at least a portion of spacer 16 therein. The combination ofinterior chambers 44 and 46 cooperate to receive the cylindricallyshaped spacer 16 and restrain movement of spacer 16 in shells 12, 14. Inthe illustrated embodiment, the junctions between interior bearing wall48 and end walls 31, 32 are curved to form an arcuate surfaces 42, 43respectively, to limit abrupt changes in the surface configuration thatmight be encountered by the spacer 16. Lower shell 14 similarly includesarcuate surfaces 47, 45 formed at the junction of bearing wall 49 andend walls 33, 34, respectively.

As shown in FIG. 3, implant 10 has an overall height in the disc spacemeasured from upper bone contacting surface 36 to lower bone contactingsurface 38. Preferably, upper shell 12 has a height H1 between bonecontacting surface 36 and its lower edge, lower shell 14 has a height H3between bone contacting surface 38 and its upper edge, and spacer 16comprises the remaining portion of the device height with a height H2extending between the edges of upper shell 12 and lower shell 14. Itwill be understood that as load is applied to the upper and lowershells, H1 and H3 will remain substantially constant while H2 will varybased on the applied load and the properties of spacer 16. Furthermore,in a preferred embodiment H2 is substantially greater than either H1 orH3, and H1 and H3 are the same. In a further embodiment, greater thanfifty percent of the spacer height is unconstrained by the upper orlower shells permitting substantial movement in the spacer to absorbcompressive loads applied to implant 10 while preserving disc motion.

The three components of top shell 12, spacer 16, and lower shell 14 arepreferably inserted into an intervertebral disc space as a single unit.The upper and lower shells may be urged towards one another to compressspacer 16. In a compressed condition, the entire implant may be loadedinto a delivery tube or sleeve which will maintain at least a portion ofthe compression during insertion. The entire implant may then bepositioned adjacent the disc space and forcibly urged into the discspace by pushing on the trailing portion of each of the shells to urgethe leading portion of implant 10 forwardly into the disc space.Preferably, a portion of the disc space has been prepared to create asubstantially cylindrical area on the adjacent vertebrae adapted toreceive the partially cylindrical portions of upper shell 12 and lowershell 14. Ribs 18, 19 extend beyond the prepared portion of the discspace and embed in adjacent bone, and the bone contacting surfaces 36,38 are intended to substantially abut the prepared disc space area toinhibit subsidence. More preferably, the prepared portion of the discspace may be limited to the area necessary to receive the length ofimplant 10 while leaving unprepared portions anterior and posterior ofthe inserted implant 10. The unprepared anterior and posterior portionsof bone may engage the end walls of the upper and lower shells to resistexpulsion of implant 10 from the disc space.

In yet a further aspect of the invention, it is contemplated that spacer16 may be comprised of hydrogels of various forms. It is contemplatedthat an alternative to or in conjunction with forcibly compressing theentire implant 10, the interior of spacer 16 can be accessed via asyringe, access port, or the like to at least partially dehydrate thehydrogel, thereby assisting in reduction of the height of implant 10between the upper and lower bone contacting surfaces 36, 38. Thereduction in implant height facilitates insertion of implant 10 into thedisc space through a smaller opening than could be utilized with anexpanded implant 10. Once positioned in the disc space, the hydrogel maybe rehydrated to thereby fully expand spacer 16 and restore implant 10to the desired height in the disc space.

Referring now to FIG. 7, there is shown a further preferred embodimentof an artificial disc implant according to another aspect of theinvention. Implant 50 includes an upper shell 52, a lower shell 54, anda spacer 56 disposed therebetween. Implant 50 includes a longitudinalaxis 51 extending therethrough. In the illustrated embodiment, uppershell 52 and lower shell 54 are substantially identical; however, it iscontemplated that there could be differences between the upper and lowershells without deviating from the spirit and scope of the presentinvention. In the description that follows, the description of uppershell 12 also applies to lower shell 14.

Referring further to FIGS. 8-9, upper shell 52 includes ribs 58 a, 58 b,58 c, 58 d, 58 e, 58 f, collectively referred to as ribs 58, and lowershell 54 includes ribs 59 a, 59 b, 59 c, 59 d, 59 e, 59 f, collectivelyreferred to as ribs 59. While straight uninterrupted ribs 58, 59 areshown in the preferred embodiment, it is contemplated that otherretention mechanisms such as barbs, interruptions, scales, etc., may beutilized to assist in retention and engagement of the upper and lowershells with the adjacent vertebral body. Upper shell 52 includes taperedleading surface 60 and tapered trailing surface 62, and lower shell 54includes tapered leading surface 61 and tapered trailing surface 63. Asexplained further below, upper shell 52 and lower shell 54 each define acavity or chamber to receive a portion of spacer 56. In the preferredembodiment, the cavity in each shell is substantially rectangular whenviewed in a top plan view.

Referring now to FIGS. 10-14, upper shell 52 will be described infurther detail, it being understood that lower shell 54 is similarlyconfigured. The upper bone contacting surface of shell 52 is comprisedof three separate regions. The first region is a partially cylindricalfirst lobe 80 having a first bone contacting surface 64 extendingconvexly along longitudinal axis 51 and interrupted by three ribs 58.The second region is also a partially cylindrical second lobe 82 havinga second bone contacting surface 68 that is substantially identical tobone contacting surface 64 extending substantially parallel with surface64 and convexly along longitudinal axis 51. Second surface 68 isinterrupted by three additional ribs 58. In the illustrated embodiment,three such ribs 58 are provided on each convex surface; however, a fewernumber or greater number of retention ribs are also contemplated. Thethird region is an intermediate portion 84 interconnecting the partiallycylindrical lobes of the first and second regions. Intermediate portion84 includes a concave bone contacting surface 66 extending between firstbone contacting surface 64 and second bone contacting surface 68.

It will be understood that the combination of surfaces 64, 66, and 68match a double-barrel insertion sleeve, including those insertionsleeves having a lumen that is figure eight or peanut shaped incross-section. Implant 50 is placed through the insertion sleeve andinto the disc space after reaming the adjacent vertebral end plates toform two substantially cylindrical holes spaced by a desired distance.The distance between the reamed holes in the end plate will determinethe size of the intermediate portion 84 between first lobe 80 and secondlobe 82. Intermediate portion 84 will be reduced in size as the reamedholes in the disc space are positioned closer to provide the appropriatespacing between convex surfaces 64 and 68. Conversely, as the distancebetween the reamed holes in the disc space increases, the intermediateportion 84 will be increased in size to provide the appropriate spacingbetween convex surfaces 64 and 68.

As shown in FIGS. 12-13, spacer retaining cavity 70 is formed by sidewalls 74 and 76, flat internal load bearing surfaces 80 and 82, a convexarcuate surface 78 extending between load bearing surface 80 and 82, andend walls 72 and 73. In a preferred embodiment, width W1 between sidewall 74 and side wall 76 is greater than the width W2 between leadingend wall 72 and trailing end wall 73. Cavity 70 has a maximum height H4.Spacer 56 has an upper surface configured for bearing contact with eachof the surfaces 80, 82, and 78 in the upper shell 52 and a lower surfaceto match corresponding cavity bearing surfaces in lower shell 54. Asshown more clearly in FIGS. 8 and 9 and as described above with respectto implant 10, greater than fifty percent of the overall height ofspacer 56 is unconstrained by the upper and lower shells. Spacer 56 alsohas opposite sidewalls 69, 71 and opposite endwalls 65, 67 orientedvertically between upper shell 52 and lower shell 54 such that spacer 56is confined entirely within upper shell 52 and lower shell 54 with noportion of spacer 56 extending outside the sidewalls and end walls ofshells 52, 54.

An alternate form of shell 52 is provided in FIG. 12a and designated as52′. Shell 52′ is identical to shell 52, except that cavity 70′ isdefined by a relatively flat surface 80′ extending between leading endwall 72, trailing end wall 73, and sidewalls 74, 76.

Referring now to FIGS. 15-17, there is shown yet a further embodiment ofan artificial disc implant according to the present invention. Implant110 includes an upper shell 112, a lower shell 114 separated by a spacer116. Upper shell 112 includes a number of ribs 118, and lower shell 114includes a number of ribs 119. The outer configuration of the upper andlower shells 112 and 114 respectively, is substantially identical to theouter configuration of the upper and lower shells 52, 54 of implant 50.Implant 110 differs from implant 50 with respect to the interior cavityadapted to engage spacer 116 and the configuration of spacer 116.

Spacer 116 includes a first substantially cylindrical lobe 130, a secondsubstantially cylindrical lobe 132, and an intermediate portion 134joining lobes 130 and 132. Preferably spacer 116 is formed as ahomogenous unit, and the end view of spacer 116, as shown in FIG. 16,has a substantially figure-eight or peanut shaped configuration.

Referring now further to FIGS. 18-22, there is shown upper shell 112, itbeing understood that lower shell 114 is substantially identical toupper shell 112, and lower shell 114 will not be further described.Upper shell 112 includes three separate regions. The first region is afirst partially cylindrical lobe 123 having a first bone contactingsurface 120 extending convexly along longitudinal axis 111 andinterrupted by three ribs 118. The second region is a second partiallycylindrical lobe 125 having a second bone contacting surface 124 that issubstantially identical to bone contacting surface 120. Second bonecontacting surface 124 extends substantially parallel with surface 120and convexly along longitudinal axis 111. Second surface 124 isinterrupted by three additional ribs 118. In the illustrated embodiment,three such ribs 118 are provided on each convex bone engaging surface;however, a fewer number or greater number of ribs are also contemplated.The third region is an intermediate portion 127 extending between andinterconnecting the first and second cylindrical lobes. Intermediateportion 127 includes a concave bone contacting surface 122 extendingbetween first surface 120 and second surface 124.

Upper shell 112 includes an interior cavity 121 having a first lobe area136 and a second lobe area 138. Interior cavity 121 is defined by afirst interior concave surface 126 extending along and substantiallyparallel to first bone contacting surface 120, an interior convexsurface 128 extending along and substantially parallel to exteriorconcave surface 122, and a second interior concave surface 129 extendingalong and substantially parallel to second bone contacting surface 124.Cavity 121 is further bounded by a leading end wall 142 and an oppositetrailing end wall 144. Insert 116 is positionable in upper shell 112 andlower shell 114 with first lobe 130 in contact with first interiorconcave surface 126, second lobe 132 in contact with second interiorconcave surface 129, and intermediate portion 134 in contact withinterior convex surface 128. As previously disclosed with respect toembodiments 10 and 50, the upper and lower shells do not constrain morethan fifty percent of the height of spacer 116.

Referring now to FIGS. 23-27, there is shown a further embodiment of anartificial disc implant according to the present invention. Implant 150includes an upper shell 152, a lower shell 154, and a spacer 156therebetween. Similar to the spacer 116 of the previously describedimplant 110, spacer 156 includes a first lobe 160 having a substantiallycylindrical configuration, a second lobe 162 having a similarsubstantially cylindrical configuration, and an intermediate portion 164joining each of the lobes. As described above with respect to implant110, the internal cavities of upper shell 152 and lower 154 are adaptedto substantially match the configuration of spacer 156. Similarly, shell152 includes matching cylindrical lobes 180 and 182 interconnected byintermediate portion 181, and lower shell 154 has cylindrical lobes 184and 186 interconnected by intermediate portion 185. It is furthercontemplated that implant 150 can have a configuration for spacer 156and upper and lower shells 152, 154 similar to that described above withrespect to implant 50.

As shown in FIGS. 25 and 27, implant 150 may further include a flexiblemembrane 158 extending between and connected to the trailing end wallsof upper shell 152 and lower shell 154, and an opposite membrane 159extending between and connected to the leading end walls of upper shell152 and lower shell 154. Membranes 158 and 159 may act to limit movementof spacer 156 between the upper and lower shells and expulsion of spacer156 therefrom. Further, membranes 158 and 159 limit movement of theupper and lower shells with respectively to one another. In a preferredembodiment, membranes 158 and 159 are composed of a braided material. Itis contemplated that the braided membranes may be substantially flexiblein tension, and outwardly flexible as spacer 156 is compressed. In afurther embodiment, a flexible membrane is provided entirely about thespacer between the upper and lower shells.

In another aspect of this embodiment, an upper flange 165 is provided onthe trailing end of upper shell 152 extending between lobes 180 and 182.In a similar manner, a lower flange 167 is provided on a trailing end oflower shell 154 extending between lobes 184 and 186. As shown moreclearly in FIG. 27, upper flange 165 includes an aperture 166 formedtherethrough at an upwardly extending angle A1 with respect to alongitudinal axis 151. A screw 170 is insertable through aperture 166 atangle A1 to threadingly engage the bony structure of the adjacentvertebral body to anchor upper shell 152 thereto. In a similar manner,lower flange 167 has a downwardly extending aperture 168 extending at anangle A2 with respect to longitudinal axis 151. A screw 172 isinsertable through aperture 168 to engage the bony structure of theadjacent vertebral body to anchor lower shell 154 thereto.

Referring now to FIGS. 28-32, there is shown an artificial disc implantaccording to another aspect of the present invention. Implant 210includes an upper shell 212, a lower shell 214, and a spacer 216therebetween. Similar to the embodiment shown in FIG. 1, the upper andlower shells are partially cylindrical and the entire implant 210 formsa substantially cylindrical structure. Upper shell 212 includes a threadpattern 218 defined on the outer surface that corresponds and alignswith a similar thread pattern 220 formed on lower shell 214. Thus,implant 210 may be threaded into a disc space with the threads engagingthe bony structure of the adjacent vertebrae. Spacer 216 has acylindrical shape and is formed of an elastomeric compound, morepreferably a hydrogel, and is retained within a cylindrical chamberformed by upper and lower shells 212, 214. Upper shell 212 defines acavity extending between end walls 220 and 222, and lower shell 214 hasa cavity extending between end walls 224 and 226. The end walls restrictmovement of spacer 216 with respect to upper shell 212 and lower shell214 and prevent expulsion of spacer 216 therefrom.

Referring now to FIG. 33, there is shown another embodiment of anartificial disc implant according to the present invention. Implant 230includes an upper shell 232, a lower shell 234, and a spacer 236therebetween. Implant 230 is substantially cylindrical with the upperand lower shells each defining partially cylindrical portions. Theexterior surfaces of upper and lower shells 232, 234 are notuninterrupted with ribs but rather are roughened to create a boneengagement surface. In addition, it is contemplated that the surfacescould be formed such that there may be at least partial bone in-growthinto the surface of the shells to assist in anchoring implant 230 in thedisc space. The surfaces may further be coated with a BMP substance orother bone growth material to enhance bone growth.

Referring now to FIG. 34, there is shown another embodiment of anartificial disc implant according to the present invention. Implant 240includes an upper shell 242, a lower shell 244, and a spacer 246therebetween. Upper shell 242 includes a single rib 248 extendinglongitudinally along implant 240, and lower shell 244 includes acorresponding single rib 250 extending longitudinally along implant 240.Ribs 248, 250 engage the bony structure of the adjacent vertebralbodies.

Referring now to FIG. 35, there is shown another embodiment of anartificial disc implant according to the present invention. Implant 260includes an upper shell 262, a lower shell 264, and a spacer 266therebetween. Upper shell 262 includes a lower edge 263 having a firstextension 268 and a second extension 272 extending therefrom towardslower shell 264. Lower shell 264 includes an upper edge 265 having aside wall recess 270 and an end wall recess 272 formed downwardlytherein. Recess 270 corresponds to the location of extension 268, andrecess 274 corresponds to the location of extension 272. Extensions andrecesses may similarly be provided on the end and side of implant 260not illustrated in FIG. 35. When implant 240 is compressed, positioningof extension 268 in recess 270 will limit movement in the direction ofarrows from A to P (anterior to posterior). In a similar fashion,positioning of extension of 272 in recess 274 will limit movement in thedirection of arrows L to R (left and right). This limits displacement ofthe upper and lower shells relative to one another, and also limitsshear stresses in spacer 266. While both extensions are shown on uppershell 262, it is contemplated that the combination of extensions andrecesses may be alternatively formed in either the upper or lower shellswithout deviating from the teaching of the present invention.Furthermore, while arcuate recesses and extensions are shown, otherconfigurations and shapes for the recesses and extensions may beutilized, including more closely engaged structures.

Referring now to FIGS. 36a, 36 b, and 37, there is shown anotherembodiment of an implant of the present invention. FIG. 36a shows across-section of an artificial disc implant 300 having upper shell 302,lower shell 304, and an intervening spacer 306. As shown in FIG. 36b,spacer 306 includes an upper arcuate surface 316 and a lower arcuatesurface 318. The upper and lower arcuate surfaces 316, 318 are adaptedto engage the interior arcuate surfaces of upper shell 302 and lowershell 304, respectively. Insert 306 further includes truncated sidewalls 308 and 309 extending between upper arcuate surface 316 and lowerarcuate surface 318. As shown in FIG. 36a, truncated side wall 308 ispositioned adjacent the gap between lower edge 313 of upper shell 302and upper edge 315 of lower shell 304, and truncated sidewall 309 ispositioned adjacent the gap between lower edge 312 of upper shell 302and upper edge 314 of lower shell 304. These gaps between upper shell302 and lower shell 304 have a distance 310 when implant 300 is in arelaxed condition and little or no compressive force is applied betweenthe upper and lower shells. As shown in FIG. 37, when significantcompressive force F is applied to the upper and lower shells 302 and304, the edges 313, 315 and the edges 312, 314 come closer together oreven into contact, limiting the overall height displacement of implant300. By providing spacer 306 with truncated side walls 308, 309 pinchingof spacer 306 between edges 312, 314 and edges 313, 315 is substantiallyavoided. Pinching is further limited or prevented by extending the edgesof the upper and lower shell laterally beyond the truncated portions ofspacer 306.

Reference will now be made to the implants shown in FIGS. 38-41. FIG. 38illustrates implant 350 having a substantially rectangular or squareupper shell 352 and a substantially rectangular or square lower shell354. A substantially rectangular or square spacer 356 is positionedbetween upper shell 352 and lower shell 354. In a similar manner, FIG.39 shows an implant 360 having rectangular or square upper and lowershells 362, 364 and a substantially rectangular or square spacer 366therebetween. Implant 360 further includes projections 368 extendingfrom upper shell 361, and lower shell 364 includes recesses 370. Therecesses 370 receive a corresponding one of the projections 368 asimplant 360 is compressed, limiting displacement of the upper and lowershells and the shear forces in spacer 366 as previously described abovewith respect to implant 260 of FIG. 35.

FIG. 40 discloses a substantially circular implant 380 according to thepresent invention. Implant 380 includes circular upper shell 382 andcircular lower shell 384, and intervening spacer 386 therebetween. In asimilar manner, FIG. 41 shows a substantially circular implant 390having circular upper shell 392 and circular lower shell 394 and acylindrical spacer 396 therebetween. Implant 390 further includesprojections 398 and recesses 399 configured to receive a correspondingone of the projections 398 as implant 390 is compressed. This limits therelative displacement of the upper and lower shells and the shear forcesin spacer 396 as previously described above with respect implant 260 ofFIG. 35.

Referring now to FIGS. 42 and 43, there is shown a further embodiment ofan artificial disc implant of the present invention. FIG. 42 illustratesimplant 400 having a partially cylindrical upper shell 402,corresponding lower shell 404, and spacer 406 therebetween. Spacer 406has a substantially cylindrical shape. Upper shell 402 includes a firstlower edge 408 and opposite second lower edge 409. Lower shell 404includes a first upper edge 410 and opposite upper edge 411. Withimplant 400 in the relaxed condition shown in FIG. 42, the lower edgesand upper edges are spaced by a distance 412. When a force is applied toimplant 400 tending to compress it as show in FIG. 43, the lower edgesand upper edges become spaced by substantially smaller distance 414. Afurther feature of implant 400 is that the radius of curvature of spacer406 is substantially less than a radius of curvature of both upper shell402 and lower shell 404, and the elastic properties of spacer 406 areselected such that the overall compression of the spacer is limitedunder the maximum expected load such that distance 414 is the closestthe edges of upper shell 402 and lower shell 404 will come toward eachother. In this manner, pinching of spacer 406 is prevented.

Referring now to FIGS. 44-45, there is shown yet a further embodiment ofan artificial disc implant. Implant 450 includes an upper shell 452, alower shell 454 and a spacer 456 therebetween. Upper shell 452 includesa bottom surface formed by a series of ridges 458 extending toward lowershell 454. In a similar manner, lower shell 454 includes an uppersurface formed by a series of ridges 460 extending toward upper shell452. The upper and lower surfaces of spacer 456 are likewise ridged tomate with the ridges and valleys formed in the bottom surface of uppershell 452 and the upper surface of lower shell 454. With implant 450 ina relaxed condition shown in FIG. 44, the lower edge 462 of upper shell452 is spaced from upper edge 464 of lower shell 454 by a distance 466.As the maximum expected compressive force is applied to implant 450, theupper edge 464 and lower edge 462 become spaced by a distance 468 thatis substantially less than relaxed distance 466. The elastic propertiesof spacer 456 are selected such that the overall compression of thespacer is limited under the maximum expected load such that distance 466is the closest the edges will come toward each other. In this manner,pinching of spacer 406 is prevented.

Utilization of implants according to the present invention will now befurther described. It will be understood that access to the disc space,disc removal, and end plate preparation are known in the art and willonly be briefly described herein. For example, procedures andinstruments useable in a posterior approach to the disc space aredisclosed in U.S. patent application Ser. No. 09/179,999, filed Oct. 27,1998, assigned to the assignee of the present invention, and apublication by Sofamor Danek ©1996 entitled “Surgical Technique usingBone Dowel Instrumentation for Posterior Approach”, each of which isincorporated herein by reference in its entirety.

Referring now to FIGS. 46 and 47, there is shown looking posteriorly adisc space 500 positioned between an upper vertebral body V1 and a lowervertebral body V2. The anterior side of the vertebral bodies isindicated by the letter “A” and the posterior side is indicated by theletter “P”. As shown in FIG. 46, two separate implants 510 and 520 areinserted into the disc space, it being understood that implants 510 and520 can be any of the implant embodiments described herein.

A first implant position may be prepared by removing disc material fromdisc space 500 and forming, by reaming, cutting, tapping or othertechnique, arcuate portion 516 in vertebral body V1. In proceduresutilizing an insertion sleeve, such as sleeve 530, a laminectomy orfacectomy can also be performed through the sleeve. Similarly, acorresponding and aligned arcuate portion 518 is formed in vertebralbody V2. Implant 510 may then be inserted with upper shell 512contacting and/or engaged in arcuate recess 516, and lower shell 514contacting and/or engaged in arcuate recess 518. The insertion sleeve530 can maintain the implant in a reduced-size configuration duringinsertion through the sleeve by providing sleeve 530 with a channel 532having a size that is less than the relaxed size of implant 510. Theintervening elastic spacer 519 is thereby held securely between theupper vertebral body V1 and the lower vertebral body V2. A similarinstallation is performed with respect to implant 520 with upper shell522 securely contacting and/or engaged in upper vertebral body V1 andlower shell 524 contacting and/or engaged in lower vertebral body V2.

As shown in FIG. 47, portions of bony material can remain anteriorly andposteriorly of implant 510 to countersink implant 510 in the disc spaceand further resist expulsion from the disc space. Such a placement oftwo separate implants in the disc space as illustrated is more typicallyperformed during procedures that utilize a posterior approach to thedisc space. Implants 510 and 520 can also be inserted into the discspace via a posterior approach through single barrel a tube or insertionsleeve 530 via pushing or threading the implants into position throughsleeve 530. By inserting two implants in the disc space, each implantwill act independently to provide three degrees of motion, while theupper and lower shells protect the spacer from excessive wear orexpulsion.

Referring now to FIG. 48, there is shown a disc space 610 betweenvertebral bodies and an implant 612 configured for insertion through adouble-barrel insertion sleeve 614. Implant 612 can be any one of theimplants 50, 110 or 150 described above. Further, implant 612 caninclude any structure with a configuration that matches theconfiguration of interior channel 616 of sleeve 614. The procedure shownin FIG. 48 is performed by an anterior-lateral approach to the discspace 610, although anterior and lateral approaches are contemplated aswell. Procedures and instruments useable in an anterior approach aredisclosed in U.S. patent application Ser. No. 09/287,917, filed Apr. 7,1999, assigned to the assignee of the present invention, and apublication by Sofamor Danek ©1996 entitled “Surgical Technique usingBone Dowel Instrumentation for Anterior Approach”, each of which isincorporated herein by reference in its entirety.

Interior channel 616 of insertion sleeve 614 and implant 612 may besized such that implant 612 is maintained in an at least partiallycompressed condition during insertion, allowing insertion of implant 612into the disc space in a reduced height state. It will be understoodthat the end plates of the adjacent vertebral bodies to disc space 610are prepared to receive implant 612 prior to implant insertion.Techniques for shaping the vertebral end plates to conform to thegeometry of devices positioned in the disc space are well-known in theart and will not be further described herein. It is preferred that thelocations for the cylindrical lobes of implant 612 are prepared byreaming the disc space, and further that the reamed implant locationwill allow the implant to be countersunk into the vertebral bodies toprevent expulsion of the implant from the disc space. Once implant 612is inserted, the spacer will expand so that the upper and lower shellscontact and/or engage the vertebral endplates to maintain implant 612 indisc space 610.

Referring now to FIGS. 49a-51, various implants are shown to accommodatevarious approaches for inserting the implants into the disc space. Forthe purposes of clarity the upper and lower shells of the implants arenot illustrated in order to more clearly show the orientation andrelative sizes of the implant spacers in the disc space. It should beunderstood that the illustrated spacers could be used with any of theimplant embodiments described herein. It should be further understoodthat the implant spacers may be provided as separate components as shownin FIGS. 49a-5 b or interconnected by an intervening spacer portionextending therebetween.

Referring more specifically to FIG. 49a, there is shown an implant 700inserted via an anterior approach to the disc space. More specifically,it is contemplated an implant inserted with this approach and havingthis configuration is inserted between the L5 and S1 vertebral bodies,although other vertebral levels are also contemplated. For such anapproach, the spacers 702 in implant 700 may have a substantiallytapered or truncated trapezoidal shape, as shown in FIG. 49b, toestablish and/or maintain the appropriate lordosis between the vertebralbodies with the posterior end of the spacer smaller than the anteriorend of the spacer.

Referring now to FIG. 50a, an anterior-lateral approach to the discspace is taken to insert implant 710 in the disc space, Morespecifically, it is contemplated an implant inserted with this approachand having this configuration can have particular application betweenthe L4 and L5 vertebral bodies, although other vertebral levels are alsocontemplated. In this approach implant 710 has an anterior spacer 712with a substantially tapered or trapezoidal shape, while the posteriorspacer 714 has a substantially cylindrical shape, as shown in FIG. 50b,to establish and/or maintain the appropriate lordosis between thevertebral bodies.

Referring now to FIG. 51a, an implant 720 is positioned in the discspace by a substantially lateral approach. As shown in FIG. 51b, implant720 includes an anterior spacer 722 and a posterior spacer 724. Thedifference in diameters between spacers 722 and 724 is provided toestablish and/or maintain the appropriate lordosis between the vertebralbodies. This configuration would be particularly adapted to insertion ofimplant 720 between vertebral bodies L1 and L5, although insertion atother vertebral levels is also contemplated.

Referring now to FIGS. 52 and 53, there is shown a further embodiment ofan artificial disc implant of the present invention. Implant 800includes an upper shell 802 and a lower shell 804. Upper shell 802includes a first partially cylindrical lobe 810 interconnected with asmaller second partially cylindrical lobe 812 via intermediate portion811. Lower shell 804 similarly includes a first partially cylindricallobe 814 interconnected with a smaller second partially cylindrical lobe816 via intermediate portion 813. Intermediate portions 811, 813 can beprovided with varying sizes as needed to achieve the desired spacingbetween the first and second lobes of shells 802, 804, respectively. Anumber of ribs 818 extend from first lobes 810, 814 and a number of ribs819 extend from second lobes 812, 816. A first spacer 806 is positionedbetween first lobe 810 of upper shell 802 and first lobe 814 of lowershell 804. A second spacer 808 is positioned between second lobe 812 ofupper shell 802 and second lobe 816 of lower shell 804.

First spacer 806 has a height H4 that is greater than a height H5 ofsecond spacer 808. First spacer 806 further has a length L4 that isgreater than length L5 of second spacer 808. First lobes 810 and 814 areconfigured to accommodate first spacer 806, and first lobes 810, 814 canbe provided with endwalls to prevent spacer 806 from protruding orexpulsing therefrom. Second lobes 812 and 816 are configured toaccommodate second spacer 808, and second lobes 812, 816 can be providedwith endwalls to prevent spacer 808 from protruding or expulsingtherefrom. Implant 800 would be particularly suited in a lateralapproach to the disc space as discussed above, with first spacer 806positioned toward the anterior side of the disc space and second spacer808 positioned towards the posterior side of the disc space. In anotherembodiment, it is contemplated that an intervening portion may beprovided between first spacer 806 and second spacer 808 to connect thefirst and second spacers forming a single spacer body.

Referring now to FIGS. 54a-54 d, a method for inserting an implantthrough a double barrel sleeve via a lateral approach to the disc spacewill now be described. It is contemplated that the method uses doublebarrel sleeve 612, such as that shown in FIG. 48, to provide an implantinsertion path to the disc space. In one specific embodiment, sleeve 612includes overlapping working channel portions, such as the double barrelsleeve described in pending U.S. patent application Ser. No. 09/498,426,filed Feb. 4, 2000, which is incorporated herein by reference in itsentirety. It is further contemplated that the implant inserted accordingto this technique can be any of the above-described implants, althoughpreferably the implant is one of the embodiments having a pair ofinterconnected cylindrical lobes. It is further contemplated thataspects of the described techniques also have application with anteriorand anterior-lateral approaches.

In FIG. 54a, a starting point 902 is established with respect to thedisc lateral annulus posterior to the midline of the disc. The positionof the starting point can be confirmed with a target templatefluoroscopically or radiographically. A trephine is inserted to thestarting point to incise the disc annulus at the starting point. In FIG.54b, an anterior annulus incision 904 is made vertically in the annulus,and a distraction plug 906 inserted through the starting point incisionto distract the disc space to the desired height. In an alternate form,a second distraction plug is inserted anteriorly with respect todistraction plug 906.

In FIG. 54c, double barrel sleeve 614 is inserted over a stem (notshown) extending from distraction plug 906. Preferably, sleeve 614 hastangs 618 and 620 (FIG. 48) that are inserted into the disc space.Preferably, shorter tang 618 is positioned posteriorly and longer tang620 is positioned anteriorly. It is further contemplated that anteriortang 620 can have a height larger than posterior tang 618 to assist inestablishing lordosis. It is further contemplated that the distal end ofsleeve 614 can include inferior and superior spikes 622 that can beembedded in inferior vertebra V2 and superior vertebra V1, respectively,to hold the vertebrae at the desired spacing during the procedure.

Distraction plug 906 is removed from sleeve 614 and the disc spacereamed via a reamer inserted through the respective working channelportions of sleeve 614 to formed posterior reamed location 908 andanterior reamed location 910. If an implant similar to implant 800 isprovided, or if separate implants of differing lengths are provided, thedisc space is reamed to a greater depth through the anterior workingchannel portion to accommodate the longer implant portion. When reamingis complete, the reamer is removed, as shown in FIG. 54d, and theimplant positioned in the working channel of sleeve 614. If necessary,the implant can be compressed from its relaxed state for insertion intoworking channel 616.

As shown in FIG. 54e, implant 912 is pushed through working channel 616via an impactor and into the disc space. Positioning of the implant inthe disc space can be confirmed via fluoroscopic or radiographicinstrumentation. When the implant is in the desired position, a braidedfabric material 914 can be secured to vertebral bodies V1 and V2 acrossthe entry into the disc space to further resist implant expulsion fromthe disc space.

The present invention contemplates providing a variety of sizes andshapes of spacers for utilization with upper and lower shells to achievethe necessary angulation between vertebral bodies and to take intoaccount the surgeon's access to the disc space. Further, while the abovedescribed combinations have been disclosed herein as being applicable toparticular disc space, this is not a limitation on the use of suchdevices and uses in other manners or other disc space is contemplated asbeing within the spirit of the present invention.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character; it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. An artificial disc implant, comprising: an uppershell having a first portion with a first convex bone contactingsurface, a second portion with a second convex bone contacting surface,and a middle portion having a concave bone contacting surface extendingbetween said first portion and said second portion, said upper shellhaving a cavity formed therein; a lower shell having a first portionwith a first convex bone contacting surface, a second portion with asecond convex bone contacting surface, and a middle portion having aconcave bone contacting surface extending between said first portion andsaid second portion, said lower shell having a cavity formed therein;and a compressible spacer supporting said upper shell and said lowershell positioned in said cavities of said upper shell and said lowershell.
 2. The artificial disc implant of claim 1, wherein: said uppershell includes at least one rib extending along said first portion andat least one rib extending along said second portion; and said lowershell includes at least one rib extending along said first portion andat least one rib extending along said second portion.
 3. The artificialdisc implant of claim 2, wherein each of said ribs includes a taperedleading end and a tapered trailing end.
 4. The artificial disc implantof claim 2, wherein said ribs are continuous.
 5. The artificial discimplant of claim 1, wherein said spacer is elastic.
 6. The artificialdisc implant of claim 5, wherein said spacer is made from an elastomericcompound.
 7. The artificial disc implant of claim 5, wherein said spacerhas a hydrogel core.
 8. The artificial disc implant of claim 7, whereinsaid hydrogel core is dehydratable to reduce the implant height.
 9. Theartificial disc implant of claim 1, wherein said upper shell and saidlower shell are each made from a biocompatible material selected fromthe group consisting of: stainless steel, titanium, shape memory alloys,polymers, carbon fiber, and porous material.
 10. The artificial discimplant of claim 1, wherein: said upper shell includes a leading endwall and an opposite trailing end wall, each of said upper shell endwalls extending along said first portion, said second portion, and saidmiddle portion of said upper shell and further extending towards saidlower shell; and said lower shell includes a leading end wall and anopposite trailing end wall, each of said lower shell end walls extendingalong said first portion, said second portion, and said middle portionof said lower shell and further extending towards said upper shell,whereby said end walls of said lower shell and said end walls of saidupper shell resist expulsion of said spacer from said cavities of saidupper shell and said lower shell.
 11. The artificial disc implant ofclaim 10, wherein: said leading end and said trailing end of said uppershell are tapered; and said leading end and said trailing end of saidlower shell are tapered.
 12. The artificial disc implant of claim 1,wherein: said cavity of said upper shell is defined by an arcuatesurface that substantially parallels said first convex bone contactingsurface, said second convex bone contacting surface, and said concavebone contacting surface; and said cavity of said lower shell is definedby an arcuate surface that substantially parallels said first convexbone contacting surface, said second convex bone contacting surface, andsaid concave bone contacting surface.
 13. The artificial disc implant ofclaim 1, wherein: said upper shell includes a leading end wall and anopposite trailing end wall, said upper shell further including an upperflange at said trailing end wall extending upwardly from said concavesurface of said middle portion, said upper flange having an aperturetherethrough to receive a bone screw to engage said upper shell to anupper vertebral body; and said lower shell includes a leading end walland an opposite trailing end wall, said lower shell further including alower flange at said trailing end wall extending downwardly from saidconcave surface of said middle portion, said lower flange having anaperture therethrough to receive a bone screw to engage said lower shellto a lower vertebral body.
 14. The artificial disc implant of claim 13,wherein said upper flange aperture has a central axis that extendsupwardly and towards said leading end at an angle A1 from a longitudinalaxis of the implant, and said lower flange aperture has a central axisthat extends downwardly and towards said leading end at an angle A2 fromthe longitudinal axis of the implant.
 15. An artificial disc implant,comprising: an upper shell having a leading end, a trailing end, alongitudinal axis extending therebetween, and an arcuate upper bonecontacting surface extending between said leading end and said trailingend about said longitudinal axis, said upper shell having a cavityformed therein opposite said upper surface, said upper shell furtherincluding at least one rib extending upwardly from said upper surfaceand generally in the direction of the longitudinal axis engageable inbone of an upper vertebral endplate when positioned in the disc space; alower shell having a leading end, a trailing end, a longitudinal axisextending therebetween, and an arcuate lower bone contacting surfaceextending between said leading end and said trailing end about saidlongitudinal axis, said lower shell having a cavity formed thereinopposite said lower surface, said lower shell further including at leastone rib extending downwardly from said lower surface and generally inthe direction of the longitudinal axis engageable in bone of a lowervertebral endplate when positioned in the disc space; and a compressiblespacer positioned in said cavities of said upper shell and said lowershell.
 16. The artificial disc implant of claim 15, wherein said uppershell includes three ribs and said lower shell includes three ribs. 17.The artificial disc implant of claim 15, wherein: said leading end andsaid trailing end of said upper shell are tapered; and said leading endand said trailing end of said lower shell are tapered.
 18. Theartificial disc implant of claim 17, wherein: said leading end and saidtrailing end of said at least one rib of said upper shell are tapered;and said leading end and said trailing end of said at least one rib ofsaid lower shell are tapered.
 19. The artificial disc implant of claim15, wherein: said upper shell has a first height between said uppersurface and a lower edge of said upper shell; said lower shell has asecond height between said lower surface and an upper edge of said lowershell; and said spacer being constrained in said cavities of said uppershell and said lower shell, said spacer further having an unconstrainedheight between said lower edge of said upper shell and said upper edgeof said lower shell.
 20. The artificial disc implant of claim 19,wherein said unconstrained height of said spacer is greater than thecombination of said first height and said second height.
 21. Theartificial disc implant of claim 19, wherein said spacer includesopposite concave sidewalls extending between said lower edge of saidupper shell and said upper edge of said lower shell.
 22. An artificialdisc implant, comprising: an upper shell having a cavity defined by afirst partially cylindrical lobe, a second partially cylindrical lobe,and a middle portion extending therebetween; a lower shell having acavity defined by a first partially cylindrical lobe, a second partiallycylindrical lobe, and a middle portion extending therebetween; and acompressible spacer supporting said upper shell and said lower shellpositioned in said cavities of said upper shell and said lower shell.23. The artificial disc implant of claim 22, wherein: said upper shellincludes a leading end wall and an opposite trailing end wall, each ofsaid end walls extending between said first and second cylindricallobes; and said lower shell includes a leading end wall and an oppositetrailing end wall, each of said end walls extending between said firstand second cylindrical lobes.
 24. The artificial disc implant of claim22, wherein said spacer includes a first spacer and a second spacer,wherein said first spacer is longer than said second spacer.
 25. Theartificial disc implant of claim 24, wherein said first spacer and saidsecond spacer are interconnected by an intervening portion extendingtherebetween.
 26. The artificial disc implant of claim 22, wherein saidspacer includes a first spacer and a second spacer, wherein said firstspacer of has a height greater than a height of said second spacer. 27.The artificial disc implant of claim 26, wherein said first spacer andsaid second spacer are interconnected by an intervening portionextending therebetween.
 28. The artificial disc implant of claim 22,wherein said spacer includes a first spacer and a second spacer eachhaving a tapered height for insertion anteriorly into a disc space toestablish lordosis.
 29. The artificial disc implant of claim 22,wherein: said upper shell has a first height between an upper surfaceand a lower edge of said upper shell; said lower shell has a secondheight between a lower surface and an upper edge of said lower shell;and said spacer being partially constrained in said cavities of saidupper shell and said lower shell along said shell height, said spacerfurther having an unconstrained height between said lower edge of saidupper shell and said upper edge of said lower shell.
 30. The artificialdisc implant of claim 29, wherein said unconstrained height of saidspacer is greater than the combination of said first height and saidsecond height.
 31. The artificial disc implant of claim 22, wherein:each of said lobes of said upper shell includes at least one ribextending upwardly therefrom; and each of said lobes of said lower shellincludes at least one rib extending downwardly therefrom.
 32. Theartificial disc implant of claim 31, wherein each of said ribs iscontinuous along the length of said lobe.
 33. An artificial discimplant, comprising: an upper shell having a leading end, a trailingend, and an arcuate upper bone contacting surface extendingtherebetween, said upper shell having a cavity formed therein oppositesaid upper surface, said upper shell further including three ribsextending upwardly from said upper surface and between said leading endand said trailing end; a lower shell having a leading end, a trailingend, and an arcuate lower bone contacting surface extendingtherebetween, said lower shell having a cavity formed therein oppositesaid lower surface, said lower shell further including three ribsextending downwardly from said lower surface and between said leadingend and said trailing end; and a compressible spacer positioned in saidcavities of said upper shell and said lower shell.
 34. The artificialdisc implant of claim 33, wherein: said leading end and said trailingend of said upper shell are tapered; and said leading end and saidtrailing end of said lower shell are tapered.
 35. The artificial discimplant of claim 34, wherein: said leading end and said trailing end ofeach of said three ribs of said upper shell are tapered; and saidleading end and said trailing end of each of said three ribs of saidlower shell are tapered.
 36. The artificial disc implant of claim 33,wherein: said upper shell has a first height between said upper surfaceand a lower edge of said upper shell; said lower shell has a secondheight between said lower surface and an upper edge of said lower shell;and said spacer being constrained in said cavities of said upper shelland said lower shell, said spacer further having an unconstrained heightbetween said lower edge of said upper shell and said upper edge of saidlower shell.
 37. The artificial disc implant of claim 36, wherein saidunconstrained height of said spacer is greater than the combination ofsaid first height and said second height.
 38. The artificial discimplant of claim 36, wherein said spacer includes opposite concavesidewalls extending between said lower edge of said upper shell and saidupper edge of said lower shell.
 39. An artificial disc implant,comprising: an upper shell having a tapered leading end, a taperedtrailing end, and an arcuate upper bone contacting surface extendingtherebetween, said upper shell having a cavity formed therein oppositesaid upper surface, said upper shell further including at least one ribextending upwardly from said upper surface and between said leading endand said trailing end; a lower shell having a tapered leading end, atapered trailing end, and an arcuate lower bone contacting surfaceextending therebetween, said lower shell having a cavity formed thereinopposite said lower surface, said lower shell further including at leastone rib extending downwardly from said lower surface and between saidleading end and said trailing end; and a compressible spacer positionedin said cavities of said upper shell and said lower shell.
 40. Theartificial disc implant of claim 39, wherein said upper shell includesthree ribs and said lower shell includes three ribs.
 41. The artificialdisc implant of claim 39, wherein: said leading end and said trailingend of said at least one rib of said upper shell are tapered; and saidleading end and said trailing end of said at least one rib of said lowershell are tapered.
 42. The artificial disc implant of claim 39, wherein:said upper shell has a first height between said upper surface and alower edge of said upper shell; said lower shell has a second heightbetween said lower surface and an upper edge of said lower shell; andsaid spacer being constrained in said cavities of said upper shell andsaid lower shell, said spacer further having an unconstrained heightbetween said lower edge of said upper shell and said upper edge of saidlower shell.
 43. The artificial disc implant of claim 42, wherein saidunconstrained height of said spacer is greater than the combination ofsaid first height and said second height.
 44. The artificial discimplant of claim 42, wherein said spacer includes opposite concavesidewalls extending between said lower edge of said upper shell and saidupper edge of said lower shell.